1. Field of Invention
The invention relates generally to biomedical devices and, in particular, comprises a non-invasive and passive apparatus and method that uses sensors and signal processing techniques to monitor fetal electrocardiographic waveform (EKGf), heart rate, heart rate variability and heart vector orientation and maternal heart rate and uterine contraction noise artifacts.
2. Description of the Related Art
Though the perinatal mortality rate in the United States has decreased significantly in the past three decades, the vast majority of the current perinatal deaths are thought to be attributable to potentially preventable etiologies. Prematurity, intrauterine hypoxia, perinatal infections, and maternal complications account for 60 to 80% of perinatal losses.
Maximizing the health and well-being of the mother and fetus by appropriate medical intervention is the general goal of obstetrical care. Effective monitoring of a fetus may require continuous assessment, and is commonly performed using electronic technology. However, recent escalation of the frequency of normal births by cesarean section has called into question the validity of present monitoring techniques with respect to specificity of identifying the fetus at risk. Reducing the number of unnescessary cesarean sections and, in general, reducing the number of babies that are seriously ill at birth has been raised as a national health care priority in an effort to reduce the cost of both short- and long-term health care.
Fetal assessment in this context is intended to detect conditions that, if continued; would likely result in fetal and newborn damage or death. The condition of the fetus is reflected by the cardiovascular responses in utero and may be recognized by monitoring the fetal heart rate.
The difficulties in monitoring fetal well-being have long been recognized by the medical profession. The variable position of the fetus within the womb, surrounded by the amnion and amniotic fluids makes direct examination of the fetus impossible or very difficult using most examination techniques.
Present electronic fetal heart rate monitoring shows great sensitivity, but inadequate specificity, and poor positive predictive value in correlating fetal heart rate changes with subsequent adverse neonatal outcome. Such electronic fetal heart rate monitoring, despite these limitations, remains an integral part and standard of care in the assessment of fetal status.
Presently, the primary non-invasive fetal monitoring technique is the Doppler/tocometer. The technique is cumbersome and subject to data loss as a result of fetal and maternal movement. Typically, a Doppler transducer is placed on the mother""s abdomen in a position that focuses the ultrasound signal at the fetal heart. Should the fetus move relative to the transducer, it is highly likely that the transducer will no longer be in proper position and, thus, not record an accurate heart signal. In fact, the use of a Doppler monitor is not precise enough for reliable analysis of subtle heart rate changes.
U.S. Pat. No. 5,257,627 to Rapoport relates to a portable apparatus for the non-invasive, simultaneous, self-testing of fetal and maternal signals. The device has a signal processing means for simultaneously processing fetal heart rate and maternal input signals, and also has a communication linking means for the simultaneous transmission of the fetal heart rate and maternal input data to a remote output device. Rapoport""s device uses ultrasonic means to detect the fetal heart rate.
Other non-invasive techniques are also in use. These include the processing of electrocardiograph and electromyogram signals for determination of the fetus""s well-being.
U.S. Pat. No. 4,299,234 to Epstein et al. relates to a fetal heart rate monitor which combines electrocardiograph and electromyogram type signals to increase reliability and accuracy of the resulting heart rate information.
U.S. Pat. No. 4,781,200 to Baker relates to a self-contained, lightweight ambulatory fetal monitoring system for substantially continuous analysis of fetal well-being. The monitor includes a sensor garment which is worn by the mother and has a plurality of sensors. The sensors detect fetal heartbeats and movements of the fetus within the mother. Signals developed by the sensors are processed by signal processing equipment and analyzed by a programmable data processing unit which can be provided with a variety of analytical programs which are proposed to automatically and continuously analyze fetal well-being. The sensor belt goes around the waist of the mother, and thus obstructs the surgical field.
U.S. Pat. No. 5,042,499 to Frank et al. relates to a fetal heart rate monitor that monitors weak fetal electrocardiogram signals in the presence of strong interfering noise. Frank et al""s invention non-invasively obtains from the abdomen of a pregnant subject the fetal EKGf signal, fetal heart rate, and accurate beat-to-beat heart rate variability. An operator views the EKGf signal and optimally places the set of thoracic electrodes in an attempt to adaptively cancel the maternal EKGf signal from the signal separately derived from a variably located abdominal electrocardiograph lead. There is no uniform placement of the abdominal electrodes for all patients. Placement of such leads is dependent on prior examination by a trained medical professional to identify optimal lead orientation.
The above patents, and all patents and publications mentioned in this application, are all incorporated herein in their entireties by reference.
Evaluation of the fetal electrocardiographic waveform itself might provide increased insight into the status of the fetus. Unfortunately, direct accessibility of the fetus has limited the electrocardiogram as an indicator of well-being. During labor, after the rupture of the amniotic sac, a fetal scalp electrode may be attached to the fetus""s skin. This requires twisting a wire corkscrew electrode into the presenting part of the fetus, e.g., scalp or buttocks, via the vaginal opening.
In the absence of direct electrode contact with the fetus, a large maternal signal and the presence of electrical noise (e.g. muscle artifact) has substantially precluded recognition of the fetal electrocardiogram. The placement of a fetal scalp electrode is clearly invasive, generally less comfortable for the mother, and has associated increased risks, such as infection, to the fetus, mother, and caretakers. The issue of infection has received more attention recently with increased risks of serious bloodbome infections such as AIDS.
Regardless of the monitoring technique, critical difficulties frequently arise when there is an emergent need to transfer the monitored patient from the labor area to the operating room. The monitors are usually detached during this critical interval with the mother and her fetus unmonitored during the transfer. Reattachment to monitors in the operating room (if at all) requires additional, possibly precious time and attention. Doppler transducers, if used, are inevitably in the operative field for an emergency cesarean section. Likewise, scalp electrodes must be removed or cut and withdrawn with the baby through the abdominal incision, again increasing the risk of infection.
This established need, therefore, creates a requirement for a reliable, accurate, and noninvasive technique to monitor the electrocardiogram of the fetus. Furthermore, the technique must maintain a clear operative field, accommodate movement of the mother and fetus, and be usable for a relevant portion of gestation. Moreover, it will be very desirable for the monitor output to include the fetal electrocardiogram waveform in addition to the fetal heart rate and description of heart rate variability. Monitoring of maternal heart rate and the state of uterine contractions and noise artifacts attributable to the uterus would also be desirable.
The present invention provides a method of monitoring a fetal biopotential waveform. More particularly, the present invention provides a method for generating a fetal biopotential waveform and using the waveform components to monitor many variables including, but not limited to, the fetal heart rate, the fetal heart rate variability, and/or the fetal heart vector orientation of a fetus in a pregnant mother. The method includes the steps of measuring at least one biopotential waveform indicative of the mother""s heart beat to form a maternal waveform, measuring at least one biopotential waveform indicative of the combined maternal and fetal heart beats to form a combined biopotential waveform, and using signal processing to cancel the maternal waveform from the combined waveform to derive a fetal waveform indicative of the fetus""s biopotential electrocardiographic waveform (EKGf).
The present invention also provides an apparatus for monitoring a fetal biopotential waveform. The present invention also provides an apparatus for generating a fetal biopotential waveform and using the waveform to monitor the fetal heart rate, the fetal heart rate variability, and/or the fetal heart vector orientation of a fetus in a pregnant mother. The apparatus includes at least one sensor, e.g., an electrode, for measuring at least one biopotential waveform indicative of a maternal heart beat, at least one sensor for measuring at least one biopotential waveform indicative of the combined maternal and fetal heart beats taken from a pregnant mother, and signal processing hardware, software, or hybrid mixes that can cancel the maternal waveform from the combined waveform to form a waveform indicative of the EKGf.
The present invention non-invasively and passively measures fetal and maternal electrocardiographic and maternal electromyographic waveforms by using traditional surface electrode electrocardiographic and electromyographic techniques combined with adaptive signal processing methods to solve the problems associated with the devices/techniques described above. The invention provides patient information (e.g., fetal heart rate/variability, taking into account noise artifacts attributable to uterine contractions) that at least duplicates current clinical standards.
In particular, the invention uses, for example, suitable skin contact electrodes connected to amplifiers to acquire biopotential waveforms and form signals, preferably differential signals, indicative of the mother""s heart beat from sensors, e.g., electrodes, placed on her chest, and indicative of the combined maternal and fetal heart beats from sensors placed on the mother""s abdomen, lower back, or both, as well as electromyographic signatures indicative of noise artifacts attributable to changes in uterine tone. Maternal heart rate, heart rate variability, and respiration rate are derived from the chest signals; standard maternal EKG is derived from planar leads. Instead of differential signals, more vectors may be formed by collecting single-ended signals and creating xe2x80x9cdifferential pairsxe2x80x9d therefrom.
The sensors placed on the mother""s abdomen, lower back, or both, are preferably placed to form pairs of sensors wherein each sensor of the pair is spaced from the other and each pair is positioned in a substantially criss-crossed pattern with respect to other sensor pairs. Substantial spacing between the sensors of each sensor pair and between pairs of sensors is preferred so as to achieve a three-dimensional processing of the fetal biopotential waveform. As mentioned above, the sensors are preferably positioned to avoid blocking any surgical fields, for example, the abdominal area. By sensing the combined fetal and maternal waveforms with a multiplicity of sensors, the uniqueness of the vectors can be used to establish the vector orientation of the fetus. Preferably, the number of vectors used is sufficient to achieve a clear signal indicative of the combined fetal and maternal waveforms. If a clear enough combined signal is obtained from a single sensor, the present invention can operate using a single sensor to obtain the combined waveform.
The signals from the abdominal electrodes are divided into a plurality of channels. After data validation, an adaptive signal processing filter (ASPF) algorithm or other suitable algorithm is used to cancel the estimated maternal waveform from each channel in the abdominal electrodes, using chest signals as references. The system then selects from at least one of the resulting waveforms to serve as the reference fetal waveform, for example, the waveform with the highest peak-to-peak amplitude. Using another ASPF or other suitable algorithm, the reference waveform is then processed against the other abdominal waveforms with the maternal waveforms canceled to form an enhanced fetal signal that is a representation of the EKGf. The EKGf can subsequently be used to measure fetal heart rate and other biophysical profile parameters. Surface electromyogram (EMG) signals allow for concurrent monitoring of uterine contractions and afford improved cancellation of motion artifacts including noise attributable to skeletal muscles and uterine contractions.
The present invention provides a device that is totally non-invasive, passive and will supplant the fetal scalp electrode and, therefore, eliminate those risks of infection. In one embodiment, all signals are derived from standard EKG electrodes applied to the patient""s skin.
The present invention also provides a device with sensor placement, e.g., probe electrode placement, that is universal across the patient population. Furthermore, in embodiments of the present invention wherein sensor strips or other free floating sensors, e.g., non-adhesive, are used to contact the mother""s chest, abdomen, and/or back, the patient""s position can be rotated or reorientated relative to the sensor field. In such an embodiment, the sensors must be capable of sensing a respective waveform without the need to be adhered to the patient""s body.
The present invention also provides a device where the placement of the electrodes maintains a clear surgical field, thereby facilitating operative procedures such as cesarean section deliveries, and will not interfere with resuscitation of the mother, should either become necessary.
The present invention also provides a device that overcomes the signal loss anomaly of ultrasound devices resulting from fetal movement. There is no need to tend to the device and reposition electrodes as the fetus moves, thereby allowing health professional time and attention to be directed toward more productive patient care activities.
The present invention also provides a device that will achieve a full representation of the fetal EKGf waveform which may provide useful information about the fetal condition.
The present invention also provides a device which upon interpretation of the fetal EKGf waveform makes the subject device capable of determining the instantaneous orientation of the fetal heart vector, thereby indicating the orientation of the fetus and permitting prediction of delivery complications associated with atypical presentation.
The present invention also provides a device that routinely collects maternal EKG signals. Thus, collateral information about the well-being of the mother and possible maternal-fetal interactions are immediately available.
The present invention also provides a device that will function for an ambulatory patient, either pre-term or during prolonged labors where the patient wishes to ambulate.
The present invention also provides a device that can be used in the case of non-imminent deliveries, for example, pre-term patients who may have high risk pregnancies.
The present invention also provides a device that computes and displays a unique monitoring reading that provides a measure of the instantaneous processing performance.
The present invention also provides a device that computes and displays heart rate variability information in at least two forms: i) long term variability trend, as is available with current commercial systems; and ii) a unique measure of instantaneous variability.
The present invention also provides a means to monitor multiple gestations with no additional sensors and/or processing techniques being required.
The present invention also provides a device that routinely collects electromyographic (EMG) signals as a means for monitoring maternal uterine contractions and for providing an additional signal input for noise cancellation. In addition, the device also permits the identification and characterization of active (maternal movement) and passive (surgical manipulation, uterine contraction) maternal signals from EMG inputs useful for canceling noise artifacts to even further enhance the EKGf.